Geodesy of the Equator

The latitude of the Equator is 0° (zero degrees). The length of Earth's equator is about 40,030.2 kilometres (24,873.6 mi). To calculate the actual length of the Equator would require taking into consideration that the Equator goes up and down various mountains and hills in South America, in Africa, and on various islands. The Equator is one of the five notable circles of latitude on Earth, with the others being the two Polar Circles and the two Tropical Circles: the Tropic of Cancer and the Tropic of Capricorn. The Equator is the only line of latitude which is also a great circle. The imaginary circle obtained when the Earth's equator is projected onto the sky is called the celestial equator.

The Sun, in its seasonal apparent movement across the sky, passes directly over the Equator twice each year, at the March and September equinoxes. At the Equator, the rays of the sun are perpendicular to the surface of the earth on these dates.

Places on the Equator experience the quickest rates of sunrise and sunset in the world. They are also the only places in the world where the sun can go directly from the zenith to the nadir and from the nadir to the zenith. Such places also have a theoretical constant 12 hours of day and night throughout the year, though in practice there are variations of a few minutes due to the effects of atmospheric refraction and because sunrise and sunset are measured from the time that the edge of the Sun's disk is on the horizon, rather than the center of the disk.

The Earth bulges slightly at the Equator. It has an average diameter of 12,750 kilometres (7,922 mi), but at the Equator the diameter is approximately 43 kilometres (27 mi) greater than the polar diameter.

Locations near the Equator are theoretically good sites for spaceports, such as the Guiana Space Centre in Kourou, French Guiana, as they are already moving faster than any other point on the Earth due to the Earth's rotation, and the added velocity reduces the amount of fuel needed to launch spacecraft. Spacecraft launched in this manner must launch to the east, southeast, or northeast to take advantage of this effect.

For high precision work, the Equator is not quite as fixed as the above discussion implies. The true equatorial plane must always be perpendicular to the Earth's spin axis. Although this axis is relatively stable, its position drifts about 9.0 metres (30 ft) north-south over the course of a year. Thus, the true equator moves slightly, but this effect is only relevant in extremely precise geophysical measurements.

Equatorial seasons and climate

Near the Equator there is little distinction between summer, winter, autumn, or spring. The temperatures there are usually high year-round—with the distinct exception of parts of the Equator that cross high mountains in South America and in Africa. (See Andes Mountains and Mt. Kilimanjaro.) The temperature at the Equator can also plummet during the extensive rainstorms in many locations. In many tropical regions people identify two seasons: the wet season and the dry season. However, many places close to the Equator are on the oceans or rainy throughout the year. The seasons can vary depending on a variety of factors including elevation and proximity to an ocean. These oceanic, rainy, and mountainous locations mean that the equatorial climate is not the hottest in the world.

Despite its name, no part of Equatorial Guinea's territory lies on the Equator. However, its island of Annobón is 155 kilometres (100 mi) south of the Equator, and the rest of the country lies to the north.

Monuments and signs along the line of the GPS-calculated Equator at Museo de Sitio Intiñan, Mitad del Mundo, Ecuador. In the front there is a device for "demonstrating" the notion that water circles in different directions in different hemispheres.

Crossing the Equator

Certain navies, such as the Royal Navy and the US Navy, have a tradition of holding ceremonies on board ship to mark sailors' first crossing of the Equator. These rites of initiation have in the past been notorious for their brutality. Milder line-crossing ceremonies, typically featuring King Neptune, are also held for passengers' entertainment on some civilian ocean liners and cruise ships.

Exact length of the Equator

In two widely-used geodetic standards, the Equator is modeled as a circle whose radius is a whole number of metres. In 1976 the IAU standardized this radius as 6,378,140 metres (20,925,656 ft), subsequently refined by the IUGG to 6,378,137 metres (20,925,646 ft) and adopted in WGS-84, though the yet more recent IAU-2000 has retained the old IAU-1976 value. In either case, the length of the Equator is by definition exactly 2π times the given standard, which to the nearest millimeter is 40,075,016.686 metres (131,479,713.54 ft) in WGS-84 and 40,075,035.535 metres (131,479,775.38 ft) in IAU-1976 and IAU-2000.[1]

The geographical mile is defined as one arc minute of the Equator, and therefore has different values depending on which standard equator is used, namely 1,855.3248 metres (6,087.024 ft) or 1,855.3257 metres (6,087.027 ft) for respectively WGS-84 and IAU-2000, a difference of nearly a millimeter.

The earth is standardly modeled as a sphere flattened 0.336% along its axis. This makes the Equator 0.16% longer than a meridian (as a great circle passing through the two poles). The IUGG standard meridian is, to the nearest millimeter, 40,007,862.917 metres (131,259,392.77 ft), one arc minute of which is 1,852.216 metres (6,076.82 ft), explaining the SI standardization of the nautical mile as 1,852 metres (6,076 ft), more than 3 metres (10 ft) short of the geographical mile.

Climate

The zone of the Equator has a tropical rainforest climate, also known as an equatorial climate. Usually, its average annual temperature is around 30 °C(86 °F) during the day and 23 °C(73 °F) at night. Rainfall is very high, usually from 2,500 to 3,500 mm per year. Average precipitation days are around 200 per year and average sunshine hours are around 2000 per year.

See also

Notes

^ Although millimeter precision can be important up to the scale of a mile, it has negligible physical significance at the scale of a geographic feature such as the Equator. From a computational standpoint, however, millimeter precision or better can be valuable for maintaining consistent results when used in programs for surveying and other applications that require precise measurements. As an overly simple example, if a program were to convert back and forth between the radius and the circumference of the earth sufficiently often while maintaining precision only to a meter each time, errors might accumulate until they became noticeable.

Look at other dictionaries:

Equator — E*qua tor, n. [L. aequator one who equalizes: cf. F. [ e]quateur equator. See {Equate}.] 1. (Geog.) The imaginary great circle on the earth s surface, everywhere equally distant from the two poles, and dividing the earth s surface into two… … The Collaborative International Dictionary of English

equator — late 14c., from M.L. aequator diei et noctis equalizer of day and night (when the sun is on the celestial equator, twice annually, day and night are of equal length), agent noun from L. aequare make equal (see EQUATE (Cf. equate)). Sense of… … Etymology dictionary

equator — [ē kwāt′ər, ikwāt′ər] n. [ME &LT; ML &LT; LL aequator, lit., one who makes equal: see EQUATE] 1. an imaginary circle around the earth, equally distant at all points from both the North and South Poles: it divides the earth s surface into the… … English World dictionary

equator — ► NOUN ▪ a notional line around the earth equidistant from the poles, dividing the earth into northern and southern hemispheres. ORIGIN Latin aequator, in the phrase circulus aequator diei et noctis circle equalizing day and night … English terms dictionary

equator — /i kway teuhr/, n. 1. the great circle on a sphere or heavenly body whose plane is perpendicular to the axis, equidistant everywhere from the two poles of the sphere or heavenly body. 2. the great circle of the earth that is equidistant from the… … Universalium